1. Field of the Invention
This invention relates to the method and apparatus for the manufacture of structural foam parts, bodies or articles in a multi-stage molding operation, with the article manufactured characterized by a dense, smooth, shiny, homogeneous and relatively thin molded plastic liner or skin formed at one molding station and a relatively thicker injection molded foam plastic inner structure or core integral with the smooth solid molded liner at another molding station. The same basic plastic or resin material may be used to form the composite liner and core or different resinous materials may be used to form the composite article. The thicker molded foam plastic core may optionally be formed by compression molding.
2. Description of The Prior Art
The manufacture of composite bodies has been carried out heretofore utilizing a number of different methods and apparatuses. Such bodies have been formed utilizing a number of successive operations or in a single operation. Typical prior art patents include my own U.S. Pat. Nos. 4,067,673 entitled "Apparatus For Injection Foam Molding" dated Jan. 10, 1978 and 4,155,969 entitled "Method For Injection Molding" dated May 22, 1979. Such patents disclose a process and apparatus utilizing or consisting or a single extruder and a single injection chamber where the homogeneous and foam plastic resin are extruded and stored until shot sequentially into the same mold by hydraulic pressure. The prior art cited during the prosecution of the aforementioned patents disclose various structures and methods for forming a resinous part having generally a low-density plastic core material or filler protected by a high density substantially fluid impermeable or solid skin which provides a smooth external surface of aesthetic and protective value.
All of the methods and apparatuses which have been proposed have certain disadvantages in commercial use. Some of the problems include reproduceability of the article, non-uniformity of the outer liner or surface of the article and non-uniform density of the inner core material or structure. It has been further difficult in the past to control the skin thickness and the plastic flow in the mold and therefore the skin thickness has varied from one molding shot or cycle to the next.